The invention relates to a process for graft-polymerizing vinyl monomers onto cellulose fiber materials and to polymer-grafted cellulose fiber materials prepared according to said process.
Judging from the large number of articles which have been published in recent years it can be observed that polymer-grafted cellulose fiber materials are receiving an increasing amount of attention. Depending on their composition these polymer-grafted cellulose fiber materials may be used for a variety of applications such as in paper making and as a reinforcing component in polymer composites.
Many of the methods which have been proposed for the preparation of these polymer-modified cellulose materials, have in common that a vinyl monomer is graft-polymerized onto a cellulose fiber material in the presence of an initiator system. In some of these methods an external initiator was employed, i.e. a system wherein the initiating compound was not chemically bonded to the cellulose material. Generally such a method resulted in a mixture of polymer-grafted cellulose fiber material and a considerable amount of the corresponding homopolymer, which is of course economically not very attractive when the prime interest lies in the production of the polymer-grafted cellulose fiber material. Other methods, wherein an internal initiator was employed, generally provided a higher yield of the polymer-modified cellulose material vis-a-vis that of the homopolymer. Such internal initiators may be obtained by chemically modifying the cellulose material to contain one or more peroxide groups. However, with some of the methods used for introducing the peroxide groups in the cellulose material, a cocurrent cleavage of the cellulose macromolecules is known to occur.
A method for the preparation of cellulose fiber materials containing internal initiator groups and which does not suffer from the macromolecule cleavage phenomenon as described hereinbefore, is known from the Journal of Applied Polymer Science, Vol. 23, 241-247 (1979). According to this method, a cellulose material is reacted with monochloroacetic acid and the carboxyl groups are subsequently converted to peracid groups via a reaction with sulfuric acid and hydrogen peroxide. The resulting carboxymethyl cellulose (CMC) peroxide was employed to study the preparation of polymer-grafted cellulose materials via a thermal- or photo-initiated graft-polymerization in combination with a number of vinyl monomers. The photoinitiated graft-polymerization was found to be superior to that of the thermal-initiated method, resulting in higher grafting percentages and graft efficiency, wherein grafting percentage and graft efficiency are defined as follows: ##EQU1## However with the exception of acrylamide the monomer conversion was rather low. Also, the grafting percentage left room for improvement, with the exception of acrylamide. Another disadvantage related to the photo-initiated graft-polymerization is that it required special equipment which was restricted to small scale use.
The present invention provides a method for the preparation of polymer-grafted cellulose fiber materials which does not suffer from one or more of the disadvantages described hereinbefore.
Surprisingly it has now been found that when the preparation of polymer-grafted cellulose materials is conducted via a method which employs a CMC-peroxide, as described hereinbefore, in combination with a ferrous salt, very high grafting percentages in combination with a high graft efficiency and monomer conversion, requiring special equipment which would restrict larger scale application of the method.